Probing the effect of Ni, Co and Fe doping concentrations on the antibacterial behaviors of MgO nanoparticles

The divalent transition metal ions (Ni, Co, and Fe)-doped MgO nanoparticles were synthesized via the sol–gel method. X-ray diffraction showed the MgO pure, single cubic phase of samples at 600 °C. Field emission electron microscope showed the uniform spherical shape of samples. The magnetic behavior of Ni, Co, Fe-doped MgO system were varied with Ni, Co, Fe content (0.00, 0.01, 0.03, 0.05, 0.07). The magnetic nature of pure had changed from paramagnetic to ferromagnetic. The number of oxygen vacancies increases with increasing amounts of dopant ions that lead to an ionic charge imbalance between Ni2+/Co2+/Fe2+ and Mg2+, leading to increase magnetic properties of the samples. The magnetic nature of prepared samples makes them suitable for biomedical applications. A comparative study of the antibacterial activity of nanoparticles against the Gram-negative (E. coli) and Gram-positive bacteria (S. aureus) was performed by disc diffusion, pour plate techniques, and study surface morphology of untreated and treated bacterial cell wall. An investigation of the antibacterial activity of doped MgO nanoparticles reveals that the doped MgO nanoparticles show effective antibacterial activity against the Gram-negative (E. coli) and Gram-positive (S. aureus) bacterium. The minimum inhibitory concentration of the synthesized nanoparticles against microorganisms was recorded with 40 μg/ml, while the maximum inhibitory concentration was observed with 80 μg/ml. At a concentration of 80 μg/ml, the complete growth inhibition of the E. coli was achieved with 7% Co-doped MgO and 7% Fe-doped MgO, while bacterial growth of S. aureus was inhibited by 100% in the presence of 7% Fe-doped MgO. The present work is promising for using nanomaterials as a novel antibiotic instead of the conventional antibiotics for the treatment of infectious diseases which are caused by tested bacteria.

MgO nanoparticles. Many metal oxide nanoparticles have been developed with different crystal structures such as cubic, hexagonal, etc. Chen et al. 16 synthesized cuboid structure of MgO nanofilm via the sol-gel method and verified its antibacterial activity by the control experiments of bacterial contamination, and also demonstrated that the cuboid structure of MgO nanofilm could withstand about 2000 °C by the refractory and high temperature resistance test. Azzam et al. 17 synthesized MgO nanohexagon as a potential antibacterial platform agent against microorganisms in the water, and pointed that the hexagonal sheet of MgO with a high surface area promoted the ROS production, thereby causing cytotoxicity against bacterial cells. Yamamoto et al. 18 pointed that the change of antibacterial activity of MgO-ZnO nanocubic on E. coli was similar to those on S. aureus. Mohamed et al. 19 proved the ability of the hexagonal phase crystal of ZnO and CuO nanoparticles to inhibit several pathogens including Gram-positive and Gram-negative bacteria.
MgO nanoparticles have been primarily concerned with enhancing and developing new materials due to its fascinating optoelectronic properties, defect-induced magneto-optical properties, and broad band gap. This material can be broadly used in different fields like refractory industry, waste water treatment, remediation of toxic waste, and catalyst according to its chemical, optical, magnetic, electronic, mechanical, and thermal properties [20][21][22][23][24][25][26][27][28][29] . The foreign metal dopants in the MgO crystal lattice results in drastic changes in the properties of MgO nanoparticles. The lattice structure, grain size, band gap crystal structure, breaking and coupling of ions and atoms, and electron configuration and state might be determined utilizing the preparation pathway, which is considered a significant parameter for modifying the properties of nano-oxide materials. Consequently, the synthesis route and preparation method can change the MgO nanoparticle properties by doping a foreign element into MgO lattice depending upon the various desired applications.
MgO nanoparticles wide band gap semiconductor doped with the transition metal ions has received significant interest as they serve applications in the opto-magnetic devices and field of spintronics. MgO nanoparticles is a high hygroscopic material and easy to form diamagnetism of magnesium hydroxide , reducing ferromagnetic ordering 30 . To date, no research groups have shown the ferromagnetic behavior in a MgO bulk structure experimentally. So, the magnetic properties of MgO are still unclear. Oxygen vacancy can induce the ferromagnetic behavior of MgO nanoparticles, and hydrogen adsorption can drastically enhance the magnetic moments. The origin of ferromagnetism is still debated and demands careful investigation. MgO nanoparticles is one of the most attractive model materials to examine ferromagnetism. It has a wide band-gap rock salt structure. Moreover, it is an essential barrier material used for magnetic tunnel junctions 31 . In recent years, X-ray photoelectron and photoluminescence spectroscopies indicated that the ferromagnetic behavior in the MgO system might be attributed to the Mg vacancies. Stoneham et al. reported that the magnetism induced by intrinsic defects in MgO might be correlated to the charged defects, as the experimental results explained that cation vacancy in MgO was exhibited in both charge states: singly charged V − Mg and neutral V 0 o 32 . Kumar et al. pointed out that the low population of oxygen vacancies showed the diamagnetic behavior in MgO lattice and confirmed a strong relation between ferromagnetic nature, Refs. 33,34 oxygen vacancies, and adsorbed H species 30 . The theoretical calculations and experimental results revealed that the magnetic moment of the system could be induced by cation vacancy, but the ferromagnetism of MgO nanoparticles remains debatable.
Metal oxide nanoparticles are considered one of the newest classes of materials used in biomedical applications. Magnesium oxide nanoparticles have been used in several applications such as pharmaceutical, waste remediation, glass industry, and refractory materials [35][36][37] . MgO nanoparticles are also used in catalyst and catalyst support applications [38][39][40] , etc. Many recent studies have been concerned with the antibacterial activity and the development of new and effective antibacterial reagents 41 . Due to the increasing antibiotic resistance among bacteria, the development of active antimicrobial nanomaterials sources was essential and required. The diseasecausing bacteria can resist the multiple-drug because of using random drugs used for treating the infectious diseases. In addition, antibiotics sometimes have side effects such as hypersensitivity reactions, allergic reactions, and immune suppression 42 . Consequently, there must be an extensive study in the use of nanoparticles in biological and medical applications.
Herein, we utilized a sol-gel synthesis method to synthesize spherical shaped and cubic phase of Ni , Co, and Fe doped-MgO nanoparticles. The structural, optical, magnetic properties of MgO nanoparticles with the amount of doping ions were observed. Moreover, the effect of doping ratio on antibacterial activity has been studied. It is perhaps noteworthy that this is the first study to demonstrate that 7% of Co and Fe-doped MgO nanoparticles with the mean diameter of 13.5 nm and 10.5 nm showed the excellent ability to completely restrict the growth of infectious bacteria.

Materials and synthesis methods
Chemicals. Magnesium

Results and discussion
Characterization. Structure and surface morphology as-synthesized of Ni, Co, and Fe-doped MgO nanoparticles were analyzed by x-ray diffraction (XRD), using CuKα radiation (λ = 1.5406 Å) and covering 2θ between 20° and 80° and FE-SEM measurements. The average diameter (D) of synthesized metal oxide nanoparticles was   where D is the crystalline size, K is a constant, which is equal to 0.94, λ is the wavelength of CuKα radiation (1.5406 Å), β is the width at the half maximum of the peak (2 0 0), θ is the diffraction Bragg angle. The XRD peaks were shifted slightly to a higher angle side with an increased doping concentration of Ni, Co, and Fe. Also, we observed that the crystallite size was decreased with an increase of Ni, Co, and Fe concentrations. The decrease in the mean size of the nanocrystals indicates that Ni, Co, and Fe ions doped into the host lattice were strongly affected on the crystal lattice.
The crystallite peak width is interconnected to the crystallite size. The width of the peak varies universally with the crystallite size. In the X-ray diffraction pattern, the smaller crystallites sizes promote broader peaks in the crystal plane, the higher intensities of the crystalline peaks indicate the larger crystallite size 46,47 .
The average crystallite size of the MgO metal oxide nanoparticles was evaluated using Scherrer's formula to be ∼ 14.71 nm. The average crystallite size of Ni-doped MgO nanoparticles ranged from 14.15 to 12.43 nm in diameter, while the mean crystallite size of Co-doped MgO nanoparticles was in the range from 14.51 to 11.49 nm. The average crystallite size of Fe-doped MgO nanoparticles ranged from 13.88 to 10.57 nm. All the results of the average crystallite size of Ni, Co and Fe-doped MgO nanoparticles for different doping concentrations were tabulated in Table 1.
From Table 1, we observed that increasing the concentration of Ni, Co, and Fe reduced the size of the nanoparticles of prepared samples. The results provide evidence that the ions of Ni, Co, and Fe were successfully substituted into the structure of the MgO matrix 48 . The ionic radius of (Ni 2+ , CO 2+ , and Fe 2+ ) was close to   49 . XRD analysis demonstrated that dopants (Ni 2+ , CO 2+ , and Fe 2+ ) were tightly substituted into the Mg 2+ sites of the MgO host lattice, which was possible since the ionic radius of Ni 2+ (0.069 nm), CO 2+ (0.065 nm), and Fe 2+ (0.064 nm) were smaller than the ionic radius of Mg 2+ (0.072 nm). The reduction in the crystallite size was mainly distortion in the host lattice MgO by the hosted ions (Ni 2+ , CO 2+ , and Fe 2+ ), leading to a decrease in the nucleation and inhibiting the growth of MgO nanoparticles.
The average crystallite size values of the synthesized samples were listed in Table 1, which decreases with increasing the transition metal (Ni, Co, and Fe) dopant concentrations. The lattice size was reduced by doping, especially when the doping happened with larger atoms.
The crystallite size and dopant concentrations were tabulated for all the prepared samples. These values were plotted in Fig. 4 to depict the correlation between Ni, Co, and Fe concentrations and crystallite size. Crystallite size of Ni, Co, and Fe-doped MgO nanoparticles decreased with increasing the transition metal (Ni, Co, and Fe) dopant concentrations.
Field-emission scanning electron microscope (FE-SEM). The FE-SEM inspection of samples showed changes in the size of particles as a function of concentrations of doping ions. Figure 5a-g displays the morphology and statistical particle size distribution of Ni, Co and Fe-doped MgO. FE-SEM microstructure of samples shows homogeneous and uniform distribution of particles in a spherical-shape manner 50 . These particles are highly monodispersed and of narrow size distribution. It is also noted that all particles get agglomerated on their surface. Agglomeration of particles on the surface might have originated from the high surface energy of the synthesized nanoparticles. This is a direct result of the synthesis route applied in the present work. Figure 6a-g shows the mathematical model (Gaussian curvature) to determine the maximum probability of particle size dis-  www.nature.com/scientificreports/ tribution. The particles distribution in all samples cases were uniform, and their particle size was nearly equal to particle size in XRD data. Figure 6a illustrated that the particle size of MgO was found to be in the range between 7 and 18 nm with an average diameter of 13.5 nm. For 3% and 7% of Ni-doped MgO nanoparticles, the particles size was in the range of 6-20 nm with the mean particle diameter of 13 nm and 11 nm (see Fig. 6b,c). The average diameter of particles for (3% and 7%) Co-doped MgO was equal to 12.5 nm and 10.5 nm, respectively (Fig. 6d,e). 7-17 nm was the particle size for 3% of Co-doped MgO nanoparticles and 7% of Co-doped MgO particles ranged between 7 and 15 nm. Transition metal 3% Fe-doped MgO nanoparticles have particles size lies between 7 and 17 nm. In contrast, transition metals of 7% Fe-doped MgO nanoparticles have sizes ranging between 6 and 15 nm.   Fig. 7. Thus, the blue shift of the absorption spectrum of the prepared samples was conceivable. In contrast, the transition metal ions of (Ni 2+ , Co 2+ , and Fe 2+ ) were incorporated totally into the Mg 2+ sites of the MgO host lattice 51 .
The energy band gap of the samples is estimated by Tauc's plot Fig. 8a-c using the following Tauc's Eq. (2) 52,53 .
α is the optical absorption coefficient of the materials, hν is the photon energy, E g is the direct band gap, A is a constant, and r is equal to ½ for direct allowed transitions. In order to determine the optical band-gap values, the variation of the factor (αhν) 2 as a function of the incident photon energy (hν) was plotted in Fig. 8a-c, The intercept of the extrapolated straight-line portion of the curves to zero absorption coefficient value gives the  Table 1. From Tauc's plot, the energy band gap of pure MgO nanoparticles was about 5.45 eV. The energy band gap of pure MgO nanoparticles (5.45 eV) was smaller than the energy band gap of bulk MgO (7.8 eV). The difference in band gap energy of NPs with their bulk materials may be attributed to planar defects. On the other hand, other researchers recorded that the band gap value for MgO nanoparticles was smaller than the obtained band gap value by this work 54 . This behavior might ascribe to the quantum confinement in which the optical band gap of the manufactured materials increases with decreasing nanoparticles.
The nanomaterials with a quantum dot radius show size-dependent optical properties. This behavior results from quantum confinement effects (QCE) of the charge carrier (hole-electron) 55 . So, the confinement occurs, leading to the transition from continuous to discrete energy levels.
Generally, the size of nanoparticles could create a change in the band gap energies of the materials. The particle size of MgO nanoparticles doped with Ni, Co, and Fe were decreased with the increment of dopant concentrations, as depicted in Fig. 8a-c. The lattice size of samples was reduced due to the radius of doping transition ions, that it was possible to occur due to the ionic radius of Ni 2+ (0.069 nm), Co 2+ (0.065 nm), and Fe 2+ (0.064 nm) were smaller than the ionic radius of Mg 2+ (0.072 nm). These results signified that transition metal ions of Ni 2+ , Co 2+ , and Fe 2+ have been incorporated successfully into MgO nanoparticles by substituting Mg 2+ sites of the lattice. Moreover, the band gap energies of the used nanoparticles increased with the average crystallite size decrease. The confined dimension decreased according to the reduced particle size of the samples. The reduction in confinement dimension yields discrete energy levels of materials, whereas the band gap broadens up, ultimately resulting in increasing gap energy. As depicted in Fig. 8a-c, the energy band gap of transition metal Fe-doped MgO nanoparticles was higher than the energy band-gap of the transition metal (Ni and Co)-doped MgO nanoparticles. This result might be due to the smaller particle size of transition metal Fe-doped MgO nanoparticles as compared to the particle size of the transition metal (Ni, and Co)-doped MgO nanoparticles as recorded in Table 1.
Photoluminescence measurements. PL spectra of MgO nanoparticles doped with Ni, Co, and Fe were illustrated in Fig. 9a-c. Generally, PL emission was ascribed to the existence of vacancies (magnesium or oxygen) or maybe the presence of defects (interstitial magnesium or anti-site oxygen). The presence of vacancies in the nanostructures is responsible for creating a new energy level within the band gap, leading to the emissions  Figure 9a-c illustrated the Ni, Co, and Fe-doped MgO nanoparticles and cleared that the peaks of all samples were shifted toward the smaller wavelength. The blue shift for all samples occurred as the atomic percentage of metal doping increased due to the size quantization. With increasing the transition metal concentrations from 1 to 7%, the intensity of the emission band increased. This possible occurrence is due to decreasing particle size and thus increasing surface area upon the doping percentage compared to the pure MgO nanoparticles. The doping with metal ions of Ni, Co, and Fe smaller size than Mg ion signified more structural defects than the pure MgO nanoparticles due to the crystal lattice contraction 56 .
In the photoluminescence spectra of all prepared samples, five emission peaks are observed in Fig. 9a-c. The excitation wavelength of pure MgO nanoparticles was observed at 307 nm. Two emission peaks of pure MgO nanoparticles centered at 315.2 nm, and 343.8 nm are ascribed as ultraviolet emission, i.e., near band-edge (NBE) emission. The ultraviolet emission is related to magnesium vacancies where the energy interval between the bottom of the conduction band and the magnesium vacancy (V Mg ) level are 6.31 and 5.78 eV. Violet emission band positioned at 391.6 and 405.8 nm, the violet emission probably attributed to radiative defects correlated to trapping states existing at grain boundaries. So, the emission peaks at 391.6 and 405.8 nm appeared due to radiative transition between the valence band and trapping level. A yellow emission region peaked at 583.1 nm, and this emission was created due to the recombination of electrons with holes trapped in singly ionized oxygen vacancies.  Fig. 9a. Furthermore, the peak was shifted toward a lower wavelength with increasing Ni ion concentrations. Figure 9b presented photoluminescence (PL) spectra at room temperature of transition metal Co-doped MgO nanoparticles at different concentrations. Five peaks were observed in the photoluminescence spectra. In the transition metal, 1% Co-doped MgO nanoparticles the near band edge (NBE) emission observed at 314.6 and 340.7 nm (ultraviolet emission). Two peaks appeared at about 390.8 and 404.9 nm, corresponding to a violet emission band. Finally, the yellow emission band was observed at 582.7 nm in the visible region.     63 , which indicates that the numbers of BMPs involved overlapping polarons through oxygen vacancy defects. Magnetic properties of prepared samples were transferred from paramagnetic to ferromagnetic with dopants ions concentration. The magnetic transitions could be ascribed to the influences of the vacancy defects, either oxygen vacancies or Mg vacancies, at the surfaces of the nanocrystals and their magnetic properties. Figure 11 shows the magnetization hysteresis (M-H) curve of Ni, Co, and Fe-doped MgO nanoparticles with the different content of doping ions (0.00, 0.03, and 0.07) at room temperature. The magnetization is much larger for higher substitutions.
The significant value of saturation magnetization (Ms), remanent magnetization (Mr) and coercive field (Hc), and remanence ratio (Mr/Ms) for all synthesized samples are tabulated in Table 3. As observed, the saturation magnetization values were increased with the increase in the concentration of the dopants ions. Figure 12a shows the paramagnetic behavior of MgO nanoparticles at room temperature, where Mg metal is paramagnetic in nature despite not having any unpaired electron, as well as because of alignment in nature, while doped material shows ferromagnetic nature, which might be attributed to oxygen vacancies. While VSM measurements exhibit the room temperature ferromagnetism with varying doping concentrations. The hysteresis curve shows a remnant magnetization of MgO nanoparticles is very close to zero and zero coercive field. When the substitution of divalent metal ions increases, the ferromagnetic interactions enhance. Magnetization hysteresis (M-H) loops in Ni-doped MgO at room temperature exhibited ferromagnetic behavior as shown in Fig. 12b. The magnetic properties of MgO nanoparticles changed from paramagnetic to ferromagnetic by adding doping ions into host atoms. The magnetization of Ni-doped MgO was increased with increasing the doping concentration,  66 .
In MgO nanoparticles, the magnetic properties were found to increase with increasing Co dopant ions. The arising ferromagnetic behavior in Co-doped MgO nanoparticles might be due to the ferromagnetic exchange interaction between Co 2+ and other ions. A long-range ferromagnetic exchange Interactions could be mediated through the formation of bound magnetic polarons 67 . The presence of magnetic behavior in Co-doped MgO nanoparticles could be ascribed to the presence of intrinsic vacancies within the material. As observed in magnetic hysteresis loop Fig. 12c the magnetic behavior increased at 7% Co-doped MgO nanoparticles. Co 2+ dopant ions lead to more defects/oxygen vacancy. At room temperature, this oxygen vacancy induced the ferromagnetic behavior in Co-doped MgO nanoparticles. Depend upon the previous analysis of ferromagnetism in MgO nanoparticles is proposed that the ferromagnetism in MgO originated from (Mg vacancies ( V Mg ) or/  Figure 12d shows a hysteresis loop of Fe-doped MgO nanoparticles at room temperature. Oxygen vacancies are induced when Fe ions which are divalent, are doped in MgO lattice. A high concentration of Fe 2+ doping ions leads to more defects/oxygen vacancy ( V o ) , this V o induced ferromagnetic behavior in 3% and 7% of Fe-doped MgO nanoparticles at room temperature based on the BMP model. The magnetic behavior is increased gradually by varying Fe doping concentrations. Oxygen vacancies are mainly created. This oxygen vacancy maintains charge neutrality. In the case of Fe-doped MgO nanoparticles, the electrons are trapped through the defects when Fe 2+ ions interact with oxygen vacancy. Their interaction causes polarization, which creates the magnetic moment, which is called BMPs. Saturation magnetization and coercive field of Fe-doped MgO nanoparticles were increased with increasing doping concentration, as tabulated in Table 3. The highest value of saturation magnetization was found for 7% of Fe-doped MgO nanoparticles compared to Ni and Co-doped MgO nanoparticles. The grain size of Fe-doped MgO nanoparticles might be one of the reasons for increased saturation magnetization and coercivity of these samples more than Ni and Co-doped MgO nanoparticles. In contrast, Fe-doped MgO nanoparticles have the smallest size in particles. The smaller grain size of the powders, which have higher surface-to-volume ratios, will result in many more surface vacancies. The presence of a high oxygen vacancy in the sample gives rise to the ferromagnetic ordering, which leads to the increment of magnetization. Phokha et al., reported the presented the room temperature ferromagnetism with a maximum magnetization of 1.60 emu/g at 0.07 Fe 2+ ions 68 .
As observed, the magnetic properties of all samples were increased with the deceased in the crystallite size. The correlation between saturation magnetization (Ms), remnant magnetization (Mr), and coercive field (Hc) with the crystallite size was expressed in Fig. 13a-c. The divalent metal ions (Ni, Co, and Fe) enhanced the magnetic properties of Ni, Co, and Fe-doped MgO nanoparticles by changing the concentration of dopant. Ms, Mr, and Hc were increased with increasing doping concentration. Figure 14a-c shows the correlation between doping concentration and saturation magnetization, remnant magnetization, coercivity for the used samples.  Agar disc diffusion antibacterial activity. E. coli and S. aureus, were used in this test. Initially, liquid and solid nutrient bacterial growth media were prepared and sterilized by autoclave at 121 °C for 60 min. All the tools (flask, test tubes, Petri dishes, and needles) used in this work were sterilized in an autoclave at 121 °C for 60 min. After cooled bacteria culture, bacteria E. coli and S. aureus was inoculated on the liquid medium (nutrient broth) and then incubated overnight at 37 °C. The serial dilutions of bacterial suspension were used to obtain 10 -4 of the bacteria E. coli and S. aureus colony-forming units (CFU) per ml. Prepared solid medium (nutrient agar) and allowed to cool it, but not solidify in the flask. We poured into each sterilized petri dish. Allowed the nutrient agar to harden for 20 to 30 min at room temperature. 1 ml of 10 -4 dilution of colony-forming units (CFU) was applied to the nutrient agar plates and using a glass rod for uniformly spreading it on the surface of the medium. The sterile filter paper discs were used for investigating the minimal inhibitory concentrations (MIC) of synthesized nanoparticles. The varying doses (10 μg/ml, 20 μg/ml, and 40 μg/ml) of Ni, Co, and Fedoped MgO nanoparticles were loaded on the filter paper discs and placed over the nutrient agar surface. All assays have been done in triplicates to eliminate errors during the procedure. The processes have been done under a laminar flow hood. The solid medium (nutrient agar) without nanoparticles (untreated) and containing the same concentrations of CFU was used as blank controls under the same conditions. All the Petri dishes have been incubated overnight at 37 °C. After 24 h of incubation, the inhibition zone of the antibacterial was observed, as shown in the images of Figs. 15 and 16. www.nature.com/scientificreports/ A scale in mm was used to measure the inhibition zone diameters, which had appeared around each filter paper disc. At doses 20 and 30 μg/ml of nanoparticle suspensions, there was no inhibition zone against E. coli (gram-negative) and S. aureus (gram-positive). Still, the inhibition zone was observed around each filter paper disk against E. coli and S. aureus at dose 40 μg/ml of nanoparticle suspensions as shown in Figs. 15 and 16. The differences measured of inhibition zone diameter around the filter paper disk might be attributed to many factors such as the amount of the nanoparticle concentrations suspension, size of nanoparticles, the type of the materials, and resistance of bacteria on antibacterial. The average inhibition zone diameter at 40 μg/ml and 80 μg/ml of Ni, Co, and Fe-doped MgO nanoparticles against E. coli were listed in Tables 4 and 5, respectively, as shown in Figs. 17 and 18. The values of the average diameter of the inhibition zone against S. aureus were listed in Tables 6  and 7 and shown in Figs. 17 and 18. The results of the effect of Ni, Co, and Fe-doped MgO nanoparticles on E. coli and S. aureus at 40 μg/ml and 80 μg/ml showed better activity with gram-negative bacteria (E. coli) than gram-positive bacteria (S. aureus). These results are possible because S. aureus bacteria possess a high resistance, becoming resistant to many commonly used antibiotics. As noticeable from the readings in Tables 6 and  7, the inhibition zone diameter increased by increasing the concentration of doping ions. At 7% of transition metal (Ni, Co, and Fe)-doped MgO nanoparticles, the diameters of inhibition zone bigger than the diameter of inhibition zone for 1% of transition metal (Ni, Co, and Fe)-doped MgO nanoparticles. Moreover, 7% Fe-doped MgO nanoparticles exhibit the best bactericidal effect at a 80 μg/ml concentration.
Pour plate technique. The pour plate technique was used to count the total number of colony-forming bacteria present in the liquid media. We used the mixed samples of nanoparticles colloidal suspension and bacterial suspension in this technique. First, liquid and solid nutrient bacterial growth media were prepared and sterilized for 60 min at 121 °C in an autoclave. After using autoclave, the nutrient broth medium was kept for a few minutes under room temperature till it became cool. Then it was inoculated by E. coli and S. aureus in separate test tubes and incubated for 24 h at 37 °C. The serial dilution was prepared up to 10 -4 of CFU to reduce bacterial suspension concentration. The bacterial suspensions were inoculated using nanoparticles colloidal sus- The bacteria must achieve uniform distribution in the agar plates, so the plate should gently rotate in a circular motion. The agar is allowed to solidify for about 30 min, and then the plates are inverted for incubation. One of the Petri dishes was kept as the control without treatment to compare the treated and untreated samples. In this work, the petri dish was plated in triplicate for more accuracy in the growing bacterial colonies number (treated and untreated). All the samples were incubated overnight at 37 °C. After 24 h of incubation, the growth of bacterial colonies was counted and calculated the average in the triplicate. Lastly, to evaluate the antimicrobial properties of the samples, a comparison has been made between the numbers of growing bacterial colonies treated with the number of growing bacterial colonies that were untreated by nanoparticles. Figure 19a,b showed the effective of pure MgO and Ni, Co, and Fe-doped MgO nanoparticles with dopant concentrations of 1, 3, 5, and 7% against E. coli and S. aureus bacterium. Clearly, all undoped and Ni, Co, and Fe-doped MgO nanoparticles specimens show good antibacterial activity as compared with the blank control group. Noted that 7% Fe-doped MgO NPs exhibited excellent bacterial growth inhibition effect than pure MgO nanoparticles though there existed several discernable bacterial strains. The number of viable colonies and the efficiency of inhibiting E. coli (gram-negative) and S. aureus (gram-positive) was tabulated in Tables 8 and 9. The Colony Forming Units/ml was decreased with the increase of the concentration of transition metal (Ni, Co, and Fe), as we observed in the images of Fig. 19a,b. The results revealed that the transition metal (Ni, Co, and Fe)-doped MgO nanoparticles having suitable activities with the E. coli more than S. aureus, in which S. aureus bacteria showed a stronger resistance to MgO and transition metal (Ni, Co, and Fe)-doped MgO nanoparticles as compared to E. coli bacteria. The difference among them is that the transition metal Fe-doped MgO nanoparticles possess a higher activity with the E. coli (gram-negative) and S. aureus (gram-positive) more than the transition metal (Ni and Co)-doped MgO nanoparticles. www.nature.com/scientificreports/ Most nanomaterials have antibacterial activity ascribed to various mechanisms, such as a strong reactive oxygen species (ROS), causing DNA damage and bacterial cell membrane 69 . Thus, the Ni, Co, Fe-doped MgO nanoparticles can interact with thiol groups of essential bacteria enzymes leading to their inactivation and cell death 50 . In another potential mode of mechanisms, the results of the antibacterial action of Ni, Co, Fe-doped MgO nanoparticles can be explained through the particle facilitated transport which the smaller size of metal oxide nanoparticles flood into the gram-negative bacteria cell wall. The cell wall of gram-negative bacteria consists of a single peptidoglycan layer (peptidoglycan is only a few nanometers thick) surrounded by a unique outer membrane. In contrast, the gram-positive bacteria cell wall contains many peptidoglycans layers (peptidoglycan is 30-100 nm thick), so gram-positive bacteria try to resist Ni, Co, Fe-doped MgO nanoparticles by their cell wall. By the outer bacterial membrane, the bacterial cell wall was more exposed to nanoparticles. Through the surface of microorganisms, the interaction between metal oxide nanoparticles and the bacterial cell membrane www.nature.com/scientificreports/ is occurred due to the uniquely high surface to volume ratio of metal oxide nanoparticles. This results in the aggregation of nanoparticles on the cell surface, leading to the bacterium's death. Furthermore, many studies have signified that MgO nanoparticles have dosage-dependent antibacterial activity due to the particle size-dependent antibacterial effects. For example, jin and He reported that higher concentrations of MgO nanoparticles cause more significant bacterial inactivation 70 . In addition, Sawai proved that increasing MgO concentration leads to increasing the activity of MgO nanoparticles against E. coli 71 .
As observed in Fig. 20a-c, the CFU of E. coli and S. aureus was decreased with the increasing concentrations of transition metal (Ni, Co, and Fe). Figure 20a-c showed that 7% of the transition metal Fe-doped MgO nanoparticles having a good effect on both classes of bacteria gram-negative (E. coli) and gram-positive (S. aureus), with a comparison between all of the different concentrations of the dopant transition metal as we have seen that 7% of transition metal Fe-doped MgO nanoparticles possess good activity against the bacteria E. coli (gram-negative) more than S. aureus (gram-positive). Bacterial growth of E. coli and S. aureus were inhibited in the presence of the prepared particles at a concentration of 80 μg/ml. Figure 21 for (a) E. coli (b) S. aureus display the percentage of the inhibition efficiency of Ni, Co, Fe-doped MgO nanoparticles at 80 μg/ml. The maximum growth inhibition of bacteria was recorded with the 7% Co and Fe-doped MgO nanoparticles, whereas the inhibition efficiency of 7% Co and Fe-doped MgO nanoparticles were about 100% for E. coli and 7% Fe-doped MgO nanoparticles was 100% for S. aureus as shown in Fig. 21a,b. In the case of E. coli, the bacterial growth was inhibited by 95.95%  were also evaluated by detecting morphological changes in the E. coli cells before and after exposure to the nanoparticles. SEM was used to illustrate the interaction between bacterial cells and 5% of Ni, Co, and Fe-doped MgO nanoparticles. Figure 22 displays the images of E. coli cells either untreated (control) or treated with nanoparticles. Figure 22a shows the control cells of E. coli, which clearly had a rod-like shape with smooth intact surfaces. In contrast, the treated E. coli cells show the disintegration of the cell wall, as shown in Fig. 22b-e, the extensive membrane damage was observed for E. coli cells treated with 5% of Fe-doped MgO nanoparticles more than pure MgO, and 5% of Ni, Co doped MgO nanoparticles.   Fig. 23, SEM images demonstrated the effects of nanoparticles on the cell wall of S. aureus. SEM analysis sustains the hypothesis of membrane disruption. Figure 23a shows the spherical shape of cocci cells with smooth intact surfaces of control S. aureus cells, whereas S. aureus cells treated with pure MgO nanoparticles and 5% of Ni, Co, and Fe-doped MgO nanoparticles illustrated irregularly shaped (wrinkled/distorted morphology) with extensively damaged bacterial morphologies and lysed cells, as shown in Fig. 23b-e. It is also worth noting that 5% of Fe-doped MgO nanoparticles was able to destroy bacterial cell of S. aureus far more affectively as compared to pure MgO nanoparticles as well as 5% of Ni, Co-doped MgO nanoparticles. Therefore, all the findings of damaged bacterial cells of E. coli and S. aureus by pure and 5% of Ni, Co, and Fe-doped MgO nanoparticles were effective in penetrating the cells and disrupting their vital functions such as cell metabolism, cell division, DNA replication, etc. with the expulsion of cellular contents. The interaction between the nanoparticles and the cell wall of bacteria was changed due to doping of Ni, Co, and Fe. The bacterial growth of E. coli and S. aureus was more commendably affected by Co, and Fe-doped MgO nanostructures compared with Ni-doped MgO nanoparticles. The difference in the antibacterial activity of Ni, Co, and Fe-doped MgO nanostructures against Gram-negative and Gram-positive bacterial strains may be due to the difference in the cell wall structure of those pathogen bacteria. Earlier studies showed that various bacterial strains had considerably different infectivity and tolerance levels towards the other agents including antibiotics 72 . Also differences in the antibacterial activity might be due to the particle size or differences in particles dissolution behavior 73 . The antibacterial efficiency of pure MgO and Ni, Co, and Fe-doped MgO NPs is mainly dependent on the increased levels of reactive oxygen species (ROS). This is primarily due to the higher surface area which causes an increase in oxygen vacancies as well as the diffusion ability of the reactant molecules inside the nanoparticles. The transition metal-ions doping  www.nature.com/scientificreports/ is responsible for releasing cell content outside the cell membrane through the promoting reactive oxygen species (ROS) generation [74][75][76][77][78] . The oxidative damage of bacterial cells occurs due to the formation of ROS production (including superoxide anions, hydroxyl radicals, hydrogen radicals), which are ultimately responsible for the dismantling of cells of bacteria leading to cell death 79 .
Furthermore, the addition of the nanoparticles on the surface of the bacteria destroys cellular function and disorganization of the cell membranes. Raj et al. 80 reported that pure and Mg-doped ZnO nanostructures inhibited the growth of both bacteria (E. coli and S. aureus), and the zone of inhibition is proportional with the content of Mg doping in ZnO host lattice. Ohira et al. 81 demonstrated that the antibacterial activity was enhanced with increasing content of Zn doping in MgO lattice. They pointed out that the antibacterial activity toward S. aureus was greater than that toward E. coli bacterium. Lv et al. 82 proved that the Mg, Zn and Ce-doped CuO nanoparticles exhibited good antibacterial activity against the E. coli and S. aureus bacterium, and among them 5% Mg, 3% Zn, and 5% Ce-doped CuO nanoparticles showed the best bactericidal effect at a concentration of 0.05 mg/ml.
In the present work, we proved that the most potent antibacterial activity was achieved by 100% at a very low concentration of 80 μg/ml with 7% of Co, and Fe-doped MgO nanoparticles toward bacteria E. coli and 7% of Fe-doped MgO toward S. aureus bacterium. These types of nanoparticles achieved the complete inhibition of bacterial growth. The improved antibacterial activity of the doped MgO nanoparticles might be attributed to the synergetic effect of ROS generation and the inactivation in the bacterial cells by the binding of the Ni 2+ , Co 2+ , and Fe 2+ ions-doped MgO to the bacterial cell surface.

Summary and future remarks
In this work, the different concentrations of Ni, Co, and Fe ions were doped with MgO nanoparticles via sol-gel routes. The XRD analysis confirmed the presence of the cubic structure of Ni, Co, and Fe-doped MgO nanoparticles, which is indicated by the substitution of Ni, Co, and Fe into the MgO lattice. UV-Vis measurements suggested that the band-gap increased with increasing dopant ions. FESEM showed that all samples have spherical morphology. The particles size of the samples was calculated by the FE-SEM distribution curve, which was decreased with increasing doping concentration. PL studies pointed out the presence of oxygen vacancy defects in all the samples. Magnetic behavior of Ni, Co, and Fe-doped MgO system varied with Ni, Co, and Fe content. The paramagnetic behaviour changed into ferromagnetic for the doped samples. The magnetic character was increased with the addition of Ni, Co, and Fe ions. When Ni, Co, and Fe ions are divalent, doped in MgO lattice, oxygen vacancies are mainly created. The creation of oxygen vacancies results in enhanced saturation magnetization or ferromagnetism ordering. The magnetic nature of samples makes them suitable for a broader range of biomedical applications such as hyperthermia therapy of tumors, magnetic resonance imaging contrast agents, drug delivery, and others. The antibacterial activity of Ni, Co, and Fe-doped MgO showed more effectiveness in inhibiting the growth of bacteria E. coli as compared to bacteria S. aureus. Ni, Co, and Fe-doped MgO nanoparticles at 80 μg/ml concentration showed a remarkable maximum inhibition zone against E. coli and S. aureus, while the minimum inhibition zone was clearly remarked at 40 μg/ml. Furthermore, the effect of Ni, Co, and Fe-doped MgO nanoparticles at the concentration of 80 μg/ml confirmed that Fe-doped MgO was more efficient to restrict the bacterial cell growth than Ni, Co-doped MgO nanoparticles. This work concluded that antibacterial tests achieved the efficiency of prepared nanoparticles of 100% with 7% (Co/Fe)-doped MgO against bacteria E. coli, and 100% with 7% Fe-doped MgO against bacteria  www.nature.com/scientificreports/ S. aureus. As our findings cleared, with the smallest nanoparticles of 7% Fe-doped MgO, the bacterial growth was utterly inhibited, which might be due to the ease of penetration in the bacterial cell membrane. This proves that the nanoparticle size correlates with the growth inhibition of the bacterium. SEM images confirmed that the treated bacterial cells of E. coli and S. aureus were significantly changed and showed significant damage in the bacterial cells. Based on these results, we expect that these materials will open a new window for the researchers to give bright insights and new uses of these materials, especially in biomedical applications, such as involving in biomedical and pharmaceutical industries and controlling the use of antibiotics. In addition, due to the magnetic nature of these materials, they can act as topical drug delivery systems. So, the research perspective of these materials is critical to explore.

Data availability
All data generated or analysed during this study are included in this published article.